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OPEN Characteristics and emergency mitigation of the 2018 Laochang landslide in Tianquan County, Province, Zhuo Chen1,2, Danqing Song3* & Lihu Dong4

This paper describes a recent landslide event, which occurred at Liucheng village in Tianquan County, Sichuan Province, China, on July 15, 2018. The Laochang landslide described in this research is an outstanding and valuable reference for understanding the characteristics of such kind of landslides that are geologically similar to the landslide. The deformation characteristics of the landslide are investigated based on feld investigations, drilled boreholes, and exploratory trenches. The 225 residents of 64 households living on the fat platform were threatened by the landslide. Therefore, to guarantee the safety of human life and property becomes the primary emergency task. The anti- sliding piles were taken to stabilize the landslide and mitigate impacts caused by the landslide. Based on the analysis of the monitoring data, the efectiveness of anti-sliding piles is evaluated. The results indicate that the anti-sliding piles are efective in increasing the stability of the landslide, and this work can provide a reference for similar slope engineering projects.

Landslides represent a common geohazard in many parts of the world and became a major concern to national and local authorities­ 1,2. A large number of cases have been reported in many countries such as China, India, Vietnam, ­Japan3–8. Landslides in Southwest China are particularly widespread, threatening human activities and producing socio-economic ­losses9–15. Tianquan County is one of the landslide-prone areas in southwest Sichuan Province, China. Te number of landslides has increased markedly since the 1970s, and these landslides have caused a lot of property damages each year. According to a 2018 survey, there were more than 220 landslides in Tianquan County, including more than 20 disastrous landslides. Te frequently occurred landslides in remote mountainous areas are receiving increasing attention from many researchers, because some of them may have devastating ­efects16–23. Landslides in these areas ofen cluster regionally with substantial negative consequences, posing important engineering problems. Tere are many fac- tors such as intense or prolonged rainfall, earthquakes, and human activities (such as unplanned vertical cut, and irrigation) responsible for exacerbating hazardous situations in Tianquan County. As shown in Fig. 1, 87.39% of the recorded landslide events in the area occur between June and September, presenting a high correlation between the temporal distribution of the landslides and rainfall. During the summer seasons of 2010–2011 and 2013–2014, rainfall events afected most areas of the region, resulting in widespread landslides and foods (Fig. 2). Two major earthquakes occurred in the past 15 years: the 2008 Ms 8.0 Wenchuan earthquake (approximately 111.611 km away from the study area) and the 2013 Ms 7.0 Lushan earthquake (approximately 25.275 km away from the study area). Hovius et al. (2011) and Marc et al. (2015) found that for large earthquakes (6.6 ≤ Ms ≤ 7.6) direct efects on landslide rates can persist as long as 4–6 ­years24,25. Terefore, the delayed efect of the two large earthquakes cannot be ignored. Te repeated vibrations of earthquakes in this area increased the crack connec- tivity and weakened natural slopes, providing favorable conditions for slope failure. In recent decades, urbanization has brought human settlements within reach of landslide hazards. Gener- ally speaking, new urban areas are built in nearby areas at slightly higher elevations. However, land shortage for agriculture and construction has become a thorny problem in Southwest China, and thus some buildings have to be erected in the proximity of or within the landslide bodies. Te building damage caused by landslides has

1Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, 611756, China. 2State Key Laboratory of Hydraulic and Mountain River Engineering, College of Water Resource and Hydropower, Sichuan University, Chengdu 610065, China. 3Department of Hydraulic Engineering, State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China. 4School of Electrical Engineering, Shenyang University of Technology, Shenyang 110870, China. *email: [email protected]

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Figure 1. Graph showing the monthly distribution of the landslides and rainfall in the period 1971–2018 in Tianquan County.

Figure 2. Photos of the rainfall-triggered food and landslides in Tianquan County. (a–c) food; (d–f) landslides.

increased because of the rapid development of urbanization on landslide-prone slopes in Tianquan County. In addition, many landslides also seem to be linked with the land conversion from natural forest to dry land agri- culture in Tianquan County. Changes in land use impose an impact on the hydrological response of an area. Due to the high proportion of farmland, agricultural irrigation has become an important agent to trigger landslides. Terefore, investigation and mitigation of such landslide disasters have become a crucial aspect for guaranteeing the safety of human life and property. A typical case study from the Laochang landslide located in the eastern sector of Tianquan County is pre- sented. Te Laochang landslide directly threatened the safety of 247 residents in 64 households, with potential economic losses exceeding 12 million CNY. Tis landslide is regionally outstanding for its dimensions and well- developed landslide morphology. Presently, slow continual deformation of the Laochang landslide can be easily

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Figure 3. Bedrock and alluvial deposits of the study area.

identifed as a sign that the landslide remains unstable. Efective engineering measures should be carried out to control the continuous displacement of the landslide. Authorized by the stakeholder, detailed feld investigation, boreholes, and exploratory trenches were carried out to characterize the landslide features. Ten the stabilization measure of the landslide is presented. Te results of this study could provide useful guidance for the prevention and reduction of landslide hazard and risk in Tianquan County. Regional setting Te landslide is located at Liucheng village, Tianquan County, Sichuan Province, China, on the lef bank of the Laochang River, a tributary of the Baoxing River. Liucheng village is situated in the eastern sector of Tianquan County, about 172 km far from Chengdu city, the capital of Sichuan Province. Te study area is located in the east wing of Baoxing anticline. Te nearest active fault to the Laochang landslide is the Shuangshi fault, about 6.6 km from the landslide boundary. Te area is part of the transitional zone that stretches from the eastern margin of the Tibetan Plateau to the eastern Sichuan Basin. Te terrain is characterized by low mountains, hills, and valleys with altitude ranging from 700 to 900 m above sea level. Te exposed strata in this region include: (1) quaternary sediments, which consist of alluvial, colluvial, and residual deposits, as well as reworked landslide deposits; and (2) the Paleogene minshan group (E­ 1-2 mn), which consists of silty mudstone intercalated with thin siltstone (Fig. 3). According to the Ya’an Meteorological Bureau (http://sc.cma.gov.cn/ds/ya/) in Sichuan Province, the study area has a subtropical monsoon climate with an average annual temperature of 15.1 °C. Te mean annual rainfall from 1971 to 2018 is 1,602.7 mm, and 64.30% of the annual average rainfall occurs between June and September. Characteristics of the Laochang landslide Te Laochang landslide (30° 8′ 31.76″ N, 102° 48′ 38.11″ E) has an irregular dustpan shape with a length of 210 m, a width of 150 m, and an average thickness of 10 m (Fig. 4). Te area of the recent landslide is 3.15 × 104 ­m2, the estimated volume is 3.15 × 105 ­m3, the sliding direction is 171°, the mean slope gradient is 10°, the elevation is 753.5–791.5 m, and the relative elevation diference is 38 m. Te lower slope is comparatively gentler than the upper part. Te rural road traverses the anterior section of the landslide, and the Laochang River is situated in the leading edge of the landslide. Many houses stand around the landslide, at the foot of the slope. Te drainage ditch is built in the proximity of the landslide. Macroscopic deformations, including cracks, minor scarps, bulges, and dislocations, have been observed within the landslide body (Fig. 5). Detailed site investigations revealed fve cracks developed on the landslide surface. Tese cracks have widths of 7–30 cm, lengths of 15–63 m, vertical displacements of 12–50 cm, and observed depths of 30–80 cm, and they cause damage to several houses. Te Wenchuan earthquake exerted a very intense impact in the landslide area, and four cracks (crack 1, crack 2, crack 3 and crack 5) developed on the landslide surface were caused by this earthquake (oral information from the local residents). According to the local residents, the Lushan earthquake enlarged these cracks. Te extruding mass movement produced bulging cracks at the landslide toe (Fig. 5e). Severe structural damage was observed, resulting in the tilt of some houses (Fig. 5f–h). Te geological structure of the landslide and the potential slip surface is obtained through boreholes and exploratory trenches, as shown in Figs. 6 and 7. Figure 4 shows the locations of the boreholes and the explora- tory trenches. Te landslide structure from bottom to top is pebble, silty clay, silty clay with rubble, respectively. According to the result of boreholes and the exploratory trenches, the location of the slip surface was confrmed (Figs. 7 and 8). Te slip surface has a depth of approximately 3.8–15.8 m and is located at the interface between the superfcial deposits and the underlying bedrock. Te sliding zone is about 0.6–0.9 m thick, which is mainly composed of clay and small gravels. According to feld investigations and its deformation characteristics, the Laochang landslide can be roughly divided into two parts: the front strong deformation part and the rear weak deformation part (Figs. 4 and 8). Tensile cracks, surface bulges, and dislocations could be found in the front strong deformation part. Te area

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Figure 4. Topographic map of the landslide. Te fgure locations show the locations of Fig. 5.

and volume of the strong deformation part are approximately 2.03 × 104 ­m2 and 2.31 × 105 ­m3, respectively. Te elevation of this zone ranges from 753.5 to 773.4 m, the average slope is 7°, and the average thickness is 11.4 m. Te deformation of the landslide promotes the development of tension cracks and bulges. Several houses have suf- fered considerable damages, and the most severely damaged buildings are located at the landslide toe. Moreover, a possible area of local instability is observed near the middle part of the landslide (Figs. 4 and 8). Some small surface fssures were observed during the rainy season, but no obvious large deformation was apparent. Te elevation of the weak deformation part is between 773.4 and 791.5 m, the average thickness is 8.6 m, the average slope is 13°, the area is approximately 1.12 × 104 ­m2, and the estimated volume is 0.84 × 105 ­m3. Figure 8 shows the landslide structure and the groundwater level in the landslide body. Te groundwater in the landslide area mainly includes two types: the perched water and the phreatic water. Te perched water is mainly found in the silty clay with rubble, and the phreatic water is found primarily in the contact zone between the silty mudstone and the overlying pebble. Tensile cracks observed on the landslide surface provide a preferential fow path for the rainwater infltration. Continuous rainfall led to increased groundwater and thus decreased the efective stress in the slope. In addition, a signifcant amount of irrigation water in spring and summer

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Figure 5. Macroscopic deformations have been observed within the landslides body. (a) Crack C1. (b) Crack C2. (c) Crack C3. (d) Crack C5. Te locations of bulges and dislocations are shown in Fig. 4.

also increased the groundwater level, exerting a negative efect on slope stability. Some of the groundwater is discharged to the ground surface in the form of spring water, indicating the abundance of groundwater in the proximity of or within the landslide body. Te high groundwater level reduces the resisting force and increases the sliding force, thereby facilitating the instability and deformation. According to the local ofcials, the number of houses shows an increase from 7 houses in 1990 to 47 in 2018 in the landslide toe, indicating insufcient urban planning by the government. Slope cutting for houses, without slope support measures, caused disturbance to the slope and changed the stress distribution within the slope, and the resisting force of the anti-slip section decreased markedly. Te modifcation of the landslide surface geometry via toe removal further reduced the slope stability. Many local residents claimed that they found a large and long crack in the middle part of the landslide in the 1990s. In 1991, a geological team also found that the width of this crack has developed to 0.5 m. Te same witnesses stated that the landslide activity increased

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Figure 6. Lithological stratigraphic logs of boreholes and exploratory trenches. (a) TC-9. (b) B03. (c) B05. (d) B06. (e) TC-4. (f) TC-7.

afer the summer of 1998. Te oral information of local residents and engineers indicates that the Laochang landslide has a long deformation history. Stability analysis of the Laochang landslide In China, the limit equilibrium ­method26, preferred among many researchers, becomes the most commonly stability evaluation method of slopes due to its conceptual clarity and simple calculation process. It should be noticed, however, that this method is not appropriate when evaluating some special landslides, such as difuse landslides plastic fow, lateral spreads, and the rockslide undergoing sudden brittle fracture­ 27. Te crucial pre- requisites of this method involve three aspects: (1) the sliding plane has been determined; (2) the failure surface observes a Mohr–Coulomb failure criterion; and (3) the sliding body maintains coherent during sliding.

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Figure 7. Te structure of the sliding mass and the slip zone of the landslide exposed by exploratory trench.

Te selection of appropriate shear strength parameters is crucial for the accurate determination of slope stabil- ity. However, it is difcult to directly obtain the appropriate parameters from testing due to the heterogeneity of the rock and soil mass. Te back-calculation approach provides a relatively easy way to do this, especially when slopes are under on-going ­failure11,28. Te mechanical parameters obtained from laboratory tests provided good references for later back-calculation. When the safety factor (Fs) is 1 (limit equilibrium state of slope stability) for back-calculation, the strength parameters of the slide zone soil can be calculated using the following equations:

Fs Wi sin αi tan ϕ Wi cos αi C − (1) = L

Fs Wi sin αi CL ϕ arctan − (2) =  Wi cos αi   where Wi is the weight of the ith calculation block; L is the length of the slip surface; and αi is the dip angle of the slip surface of the ith calculation block. In practice, Chinese engineers have empirically confrmed sound relationships between the calculated safety factor and the deformation and failure features of the landslide (Table 1). Te slip surface is presented as a pol- yline, and thus the transfer coefcient method is used to calculate the Fs of the slip surface. Te Fs of the slip surface using the transfer coefcient method is calculated according to the following equations: n 1 n 1 − 1 cos sin tan − (((Wi(( rU ) αi A αi) RDi ) ϕi CiLi) ψj) Rn i 1 − − − + j i + Fs = = n 1 n 1 (3) = − sin cos − (Wi( αi A αi) TDi ) ψj Tn i 1 [ + + j i ]+ = =

T γ h L cos α sin β cos(α β ) Di = W iW i i i i − i (4) R γ h L cos α sin β sin(α β ) Di = W iW i i i i − i (5) R (W ((1 r ) cos α A sin α ) R ) tan ϕ C L n = n − U n − n − Dn n + n n (6) T (W (sin α A cos α ) T n = n n + n + Dn (7)

n 1 − ψj ψiψi 1ψi 2 ...ψn 1 = + + − (8) j i =

ψj cos(αi αi 1) sin(αi αi 1) tan ϕi 1 = − + − − + + (9)

where Ci and φi are the cohesion and friction angle of the slip surface of the ith calculation block, respectively; TDi and RDi are the forces generated by the seepage pressure; A is the earthquake acceleration; Li is the ith calculation

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Figure 8. Longitudinal geological profles: (a) 1–1′, (b) 2–2′, (c) 3–3′ (Fig. 4).

Maximal displacement of sliding Stage Surface deformation plane Status of sliding plane Safety factor Embryonic Invisible Millimeter – ≥ 1.1 Deformation evidence at scarp and Creep Centimeter At limit equilibrium state 1.0–1.1 toe Sliding Main scarp subsidence and toe bulge Decimeter–meters Complete failure 0.95–1.0 Emplacement Sedimentation, partial collapse – – ≥ 1.0

Table 1. Relationship between safety factor and developed ­stage28.

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Parameters Landslide mass Sliding zone Natural unit gravity (kN/m3) 19.6 18.7 Saturated unit gravity (kN/m3) 20.5 19.2 Cohesion in natural state (kPa) 10.3 7.6 Cohesion in saturated state (kPa) 9.6 7.1 Friction angle in natural state (°) 29 7.7 Friction angle in saturated state (°) 19.3 7.4

Table 2. Physical and mechanical parameters of landslide mass and sliding zone soil.

Typical sections Anti-sliding pile Scenarios Safety factor Stable state Normal 1.03 Basic stable No Rainstorm 0.86 Unstable Earthquake 0.97 Unstable 1–1′ Normal 1.15 Basic stable Yes Rainstorm 1.07 Basic stable Earthquake 1.11 Basic stable Normal 1.02 Basic stable No Rainstorm 0.85 Unstable Earthquake 0.95 Unstable 2–2′ Normal 1.14 Basic stable Yes Rainstorm 1.06 Basic stable Earthquake 1.10 Basic stable Normal 1.05 Basic stable No Rainstorm 0.88 Unstable Earthquake 0.98 Unstable 3–3′ Normal 1.17 Basic stable Yes Rainstorm 1.08 Basic stable Earthquake 1.12 Basic stable

Table 3. Safety factors of the Laochang landslide under diferent scenarios.

block slip surface length; βi is the groundwater fow direction of ith calculation block; rU is the pore pressure ratio; γW is the unit weight of water; hiW is the groundwater level of the ith calculation block; ψj is the transfer coefcient of the ith calculation block to the (i + 1)th block; i = 1, 2, …, n-1; and n is the total number of the soil blocks. Tree cross-sections, including 1–1′, 2–2′, and 3–3′, are used in the stability analysis of the Laochang land- slide. Te average water table heights of the three sections are 2.54 m, 5.07 m, and 2.67 m, respectively. Te peak ground acceleration (PGA) of 0.15 g is used for pseudo-static analysis. Te physical and mechanical parameters obtained from laboratory tests are listed in Table 2. Te safety factors of the Laochang landslide were calculated under three scenarios (Table 3), indicating that the landslide without an anti-sliding pile would become unstable under both strong rainfall and earthquake conditions. Engineering control measure Based on fndings on the landslide characteristics, the main engineering control measure for stabilizing the landslide is to install anti-sliding piles because there already exists a drainage system in the proximity of the landslide (Fig. 4). Anti-sliding piles, preferred by many engineers, are considered as an efective remedial measure in landslide control projects­ 29,30. Te anti-sliding piles provide resisting forces to maintain the slope stability, its retaining efect is prominent, its structure is simple, and its design theory is relatively mature. Te design calculations of the anti-sliding pile involve four ­aspects31–33: (1) calculation of the thrust force; (2) calculation of the pile stability; (3) calculation of the internal force; and (4) calculation of the longitudinal bar and stirrup. Te thrust force acting on the cantilever part of the anti-sliding pile above the slip surface can be expressed as follows:

Pi Pi 1 ψ Ks Ti Ri (10) = − × + × − T W (sin α A cos α ) γ h L cos α sin β cos(α β ) i = i i + i + W iW i i i i − i (11)

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R (W (cos α A sin α ) γ h L γ h L cos α sin β sin(α β )) tan ϕ C L i = i i − i − W iW i − W iW i i i i − i i + i i (12) cos sin tan ψ (αi 1 αi) (αi 1 αi) ϕi (13) = − − − − −

where Pi is the thrust force of the ith calculation block; Pi-1 is the residual sliding force of the ith calculation block; Ks is the desired safety factor; Ti is the sliding force; and Ri is the resisting force. Te calculation on the pile stability can be obtained as follows: σmax P (σ σ ) ≤ r × p − a (14)

where σmax is the maximum lateral pressure of the embedded part; Ρr is the reduction factor; σp is the passive earth pressure; and σa is the active earth pressure. Te calculation of the internal force for the pile is given by:

K m(y y0)n = + (15) where K is the elastic resistance coefcient of the foundation; m is the proportion coefcient; y is the distance between the embedded part and the slip surface; y0 is a constant related to geotechnical properties; and n is a geotechnical material constant. Te calculation of the longitudinal bar and stirrup for the pile can be expressed as follows: K1M As (16) = γsfyh0

1 √1 2as γ + − (17) s = 2

K1M as 2 (18) = fcmbh0

Asv Vcs 0.07fcbh0 1.5fyv h0 (19) = + S

K2V 0.25f bh0 ≤ c (20)

where As is the cross-sectional area of the longitudinal bar; M is the design bending moment of the anti-sliding pile; fy is the design value of the tensile strength of the longitudinal bar; fcm is the design value of the compressive strength of concrete; h0 is the efective height of the pile section; b is the width of the pile section; K1 is the desired safety factor of the bending strength of the pile; V is the design value of the shear force of the pile; Vcs is the shear bearing capacity of concrete and stirrup of the pile section; fc is the design value of the concentric axial compres- sion of concrete; fyv is the stirrups’ tensile characteristic strength; Asv is the cross-sectional area of stirrups cross- ing the pile section; S is the stirrup spacing; and K2 is the desired safety factor of the shear strength of the pile. A total of 25 anti-sliding piles, with a rectangular cross-section of 1.2 × 1.5 m and a length of 10–15 m, are constructed in the middle-lower section of the Laochang landslide (Figs. 9, 10, 11, 12). Te center distance between the two adjacent piles is 5.0 m, and the embedment depth of each pile into the bedrock is 4.5–6.5 m. Te Laochang landslide with anti-sliding piles would become stable under rainstorm or earthquake conditions (Table 3). Figure 13 shows that anti-sliding piles signifcantly reduced the rates of surface displacement. For the monitor- ing point J3, the rate of surface displacement afer construction was 0.079 mm/day, in contrast to 0.244 mm/day before the construction of anti-sliding piles. For the monitoring point J2, the rate of surface displacement afer the construction of anti-sliding piles was 0.068 mm/day, compared with 0.173 mm/day before construction. Before the construction of anti-sliding piles, surface displacement increased rapidly during the rainy season. Afer the construction of anti-sliding piles, no signifcant, sudden increased displacement was observed during the rainy season. Te displacement curves were relatively fat and the rates of surface displacement were also small. Tis indicated that anti-sliding piles had a remarkable efect on slope stability. From Fig. 13b, the displacement curves of the two monitoring points (J2 and J3) increased rapidly from November 2018 to January 2019. Te main reason for this increase was the disturbance of the engineering construction between the two rows of the anti-slide piles.

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Figure 9. Longitudinal geological profle 4–4′ (Fig. 4).

To further determine the stability of the slope, it was necessary to conduct a deep displacement monitoring project before and afer the construction of anti-sliding piles. Tree inclinometers, including I1, I2, and I3, were installed along the sliding direction (Fig. 4). Te displacements in the lower part of the landslide (I3) were larger than that in the middle (I2) and upper (I1) areas (Fig. 14a–c), indicating that the deformation of the lower part of the landslide is more pronounced than the deformations of other parts. As shown in Fig. 14d–f, the slope reinforced by the anti-slide piles was in a stable state, thereby indicating the positive efect of the anti-slide piles. For the inclinometer I3, the displacement rate afer the construction of anti-sliding piles was 0.015 mm/ day, compared with 0.236 mm/day before construction. For I2, the displacement rate afer construction was 0.015 mm/day, in contrast to 0.114 mm/day before the construction of anti-sliding piles. For I1, the displacement rate decreased from 0.108 to 0.014 mm/day afer the construction of anti-sliding piles. Both surface and deep displacement increases were signifcantly reduced for all the monitoring points by the presence of anti-sliding piles. Te reinforcement efect of anti-slide piles on the slope deformation was obviously positive, and the slope body gradually became stable. Conclusions Tis paper reported that the Laochang landslide occurred on July 15, 2018, in Tianquan County, Sichuan Prov- ince, China. Ground investigations, drilled boreholes, exploratory trenches, and displacement monitoring were carried out to investigate the landslide characteristics and to analyze the engineering control measure. Te conclusions are as follows:

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Figure 10. Longitudinal geological profle 5–5′ (Fig. 4).

1. Te Laochang landslide has experienced slow creep deformation in the last 30 years and is a slow-moving landslide in the low mountains area, with a volume of 3.15 × 105 ­m3. Te slip surface was mainly the interface between the superfcial deposits and the weathered bedrock strata. Te deformation of the Laochang land- slide could be divided into two parts: the front strong deformation part with obvious deformation (such as tension cracks and bulges) and the rear weak deformation part with no obvious large deformation but some small surface fssures formed during the rainy season. 2. At present, the landslide still remains unstable, potentially endangering the lives and properties of residents downslope. Terefore, the main engineering control measure such as slope stabilizing piles was installed to strengthen the sliding body above the slip surface by placing the piles embedded into the bedrock. Afer the construction of the anti-sliding piles, the surface and deep displacement of the landslide was reduced signifcantly for all the monitoring points, indicating the efectiveness of anti-sliding piles in reducing the risk of the landslide.

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Figure 11. Longitudinal geological profle 6–6′ (Fig. 4).

Figure 12. Longitudinal geological profle 7–7′ (Fig. 4).

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Figure 13. Surface displacement monitoring result: (a) before the construction of anti-sliding piles; (b) afer the construction of anti-sliding piles (Fig. 4).

Figure 14. Deep displacement obtained by inclinometers: (a–c) before the construction of anti-sliding piles; (d–f) afer the construction of anti-sliding piles (Fig. 4).

Data availability Te raw/processed data required to reproduce these fndings cannot be shared at this time as the data also forms part of an ongoing study.

Received: 14 October 2020; Accepted: 5 January 2021

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Competing interests Te authors declare no competing interests.

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